The present invention relates to an in situ wafer cleaning process wherein group I and II metals such as sodium, potassium, calcium and magnesium are removed from the semiconductor wafer surface with a vaporous combustion product such as hydrochloric acid in a low pressure furnace during the wafer manufacturing process. The present invention also relates to an apparatus for cleaning semiconductor wafers utilizing a low pressure external combustion reactor for combusting a mixture of a halogenated hydrocarbon and oxygen to produce the stream of vaporous combustion product used to clean the semiconductor wafers.
It is well known and understood in the semiconductor industry that occupants in the cleanroom contribute "biological metals" into the cleanroom environment. These contaminants are generally group I and II metals such as sodium, potassium, calcium and magnesium. The presence of these metals in the cleanroom environment may lead to detectable levels of these metals on the surfaces of semiconductor wafers, and may cause decreased device performance in the wafer.
The detrimental effects of group I and II metals on device performance are well known in the art and result from interactions between the silicon lattice and the impurity. Sodium moves quickly through the silicon lattice at high temperatures and can cause undesired changes in bulk electrical properties. Potassium can also detrimentally effect bulk electrical properties and can further interfere and cause an inconsistent growth rate of gate oxides. Both calcium and magnesium have similar undesirable effects on silicon wafers.
The use of vaporous hydrochloric acid to remove group I and II metals as well as other impurities such as transition metals from the surfaces of semiconductor wafers is known in the art. Meador et. al. in U.S. Pat. No. 4,544,416 disclose a method for cleaning a silicon dioxide layer on a semiconductor wafer wherein the wafer is placed in a furnace tube having an atmosphere of steam or dry oxygen and held at a temperature of about 600.degree. C. Thereafter vaporous hydrochloric acid is added to produce water and chlorine, and the chlorine getters ionic contaminants from the wafer surface.
However, the use of bottled pure hydrochloric acid gas inside a furnace or furnace tube has many disadvantages such as cost and difficulty in management and containment. Furthermore, vaporous hydrochloric acid in the presence of moisture can corrode equipment and lead to further contamination of the wafer with transition metals and a degradation in device performance.
Due to the disadvantages of bottled hydrochloric acid gas, alternative sources for the production of chlorine ions have been sought. DeBusk et. al. studied the use of trans-1,2-dichloroethylene (t-DCE) as a chlorine generating species to clean semiconductor wafers contaminated with iron. (DeBusk, Lagendijk and Porter, "Investigating a Trans-dichloroethylene Vapor Cleaning Process", MICRO, pp. 39-43, September, 1995). In an external torch system held at atmospheric pressure, bubbled t-DCE was carried into a mixing chamber by nitrogen and combusted with oxygen at around 900.degree. C. to produce vaporous hydrochloric acid. The vaporous hydrochloric acid was subsequently injected into a furnace at atmospheric pressure and passed over a series of wafers. DeBusk et. al. determined that a t-DCE vapor cleaning process removed iron efficiently.
However, a need exists in the semiconductor industry for an in situ cleaning process which can be used during wafer processing at reduced pressures without introducing separate steps into the process. Currently, commercially available systems are incapable of tightly controlling the amounts of unreacted hydrocarbon and oxygen which may ultimately contact and contaminate the wafer as these systems require a large amount of a carrier gas to introduce the hydrocarbon into the system. Unreacted components become important, for example, when a polysilicon layer is deposited on a wafer in a LPCVD reactor subsequent to the wafer cleaning process. Unreacted oxygen can cause unwanted oxide layer formation on the wafer, while unreacted hydrocarbon on the wafer surface can result in the formation of non-uniform silicon grain size.
Furthermore, a need exists for a combustion reactor capable of producing combustion products at low pressures. Currently, combustion reactors utilized for burning liquid halogenated hydrocarbons with oxygen introduce the hydrocarbon by bubbling an inert gas such as nitrogen through the liquid hydrocarbon thus creating vapors containing nitrogen and the hydrocarbon. However, with a low pressure application, the amount of carrier gas required to create sufficient vaporous hydrocarbon is too much for the vacuum pumps to handle and maintain low pressures. Current reactors simply are not compatible with low pressure applications. Also, with commercially available reactors it has proven difficult, if not impossible, to tightly control the amount of hydrocarbon in the vapor phase with the carrier gas. This lack of control of the amount of vaporous hydrocarbon can lead to unacceptable levels of unreacted starting materials which may subsequently contact and contaminate the wafer surface.